Silicon nitride is of great interest for structural and electronic applications because of its excellent high temperature strength, good thermal shock resistance, good wear resistance and chemical inertness. Silicon nitride materials have been traditionally fabricated by one of three methods: (1) the direct nitridation of silicon powder at high temperatures, (2) densification of commercially available silicon nitride powder at high temperatures by the addition of metal oxide powders known as sintering aids (sintering aids promote densification of the silicon nitride by providing a liquid phase during sintering) and (3) densification of commercially available silicon nitride powder at high temperatures and pressures (hot pressing). These silicon nitride materials have been used in structural applications such as cutting tools and gas turbines. However, these materials suffer brittle fracture, which limits their usefulness in commercial applications.
The brittleness of a ceramic can be decreased by introducing a secondary phase such as ceramic whiskers, fibers or particulates. These additives act as toughening agents by absorbing energy during crack propagation. The mechanical properties of a ceramic can also be improved by introducing a compatible metal phase. Such ceramic/metal composites or "cermets" provide the strength and ductility of a metal combined with the hardness and chemical inertness of a ceramic. For example, U.S. Pat. No. 4,332,909 discloses a composition consisting of silicon nitride, at least one oxide of yttrium, scandium, cerium, lanthanum and the metal oxides of the lanthanide series, and at least one metal or metal oxide of iron, nickel and cobalt. When large amounts of the metals or metal oxides are used, they form a net-like continuous structure in the silicon nitride. U.S. Pat. No. 3,409,417 discloses a dense, refractory composition of silicon nitride and a sintering aid, bonded with iron, cobalt, nickel, chromium, rhenium, tungsten, molybdenum or their alloys. The composition can be used to prepare cutting tools Japanese unexamined patent application 58/64,270 describes a silicon nitride sintered product prepared from a mixed powder of at least one metal oxide, at least one metal silicide and the remainder silicon nitride. The metal silicide decomposes during sintering to produce a metal layer on the surface of the product. Russian patent 485,999 discloses a cermet composition comprising 16-20 wt. % Fe, 12-18% Si.sub.3 N.sub.4 filamentary crystals and the balance ZrO.sub.2, prepared by hot pressing a powder mixture. While cermets such as those shown in these prior art references offer distinct mechanical property advantages over single component ceramics at ambient and modestly high temperatures, their toughness, wear resistance and strength are often degraded sufficiently at temperatures greater than 1000.degree. C. to render them unusable in a variety of structural applications.